Integrated optics reflectometer

ABSTRACT

An apparatus includes a laser source configured to output laser light at a target frequency, and a measurement unit configured to measure a deviation between an actual frequency outputted by the laser source at a current period of time and the target frequency of the laser source. The apparatus includes a feedback control unit configured to, based on the measured deviation between the actual and target frequencies, control the laser source to maintain a constant frequency of laser output from the laser source so that the frequency of laser light transmitted from the laser source is adjusted to the target frequency. The feedback control unit can control the laser source to maintain a linear rate of change in the frequency of its laser light output, and compensate for characteristics of the measurement unit utilized for frequency measurement. A method is provided for performing the feedback control of the laser source.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/610,533, filed on Mar. 14, 2012, and to U.S. application Ser. No.13/829,728 filed Mar. 14, 2013. The entire contents of U.S. ProvisionalApplication No. 61/610,533 and U.S. application Ser. No. 13/829,728 arehereby incorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

At least part of the present disclosure was conceived or first actuallyreduced to practice under a contract with the U.S. Department of Energy.

FIELD

The present disclosure relates to an apparatus and method for testingthe optical integrity of fiber optic (FO) installations. Moreparticularly, the present disclosure relates to an integrated opticsreflectometer and associated method which can test the optical integrityof FO installations at high resolution (e.g., less than 1 cm, such asless than 2 mm) at a range of one or more kilometers inexpensively andeffectively in a single sweep.

BACKGROUND

Optical communication techniques, including fiber optics and lasers, arethe workhorses of the Internet and high-capacity computing. Meeting thecomputing and telecommunications needs of the next decades will requireadvances across a broad front of research and development, includingoptical signal generation, transmission, switching and routing, as wellas intelligent and seamless networking. Although institutions andcompanies have access to such rapidly growing, high-speed globaltelecommunication networks, the infrastructure is not yet in place toprovide the individual user access that fully exploits the power oflight in FO installations.

Known FO diagnostic instruments consist of two expensive categories: 1)long-range units with relatively low resolution for telecommunicationsand large data networks, where such units generally use optical timedomain reflectometery (OTDR) and provide tens of km of span with aresolution of 0.1 to 1 m; and 2) very-high resolution laboratoryinstruments which utilize optical frequency domain reflectometery (OFDR)providing less than 1 mm resolution.

As used herein, the terms “coherent” or “coherence” mean a uniformwavelength (or frequency). Thus, the term “coherence length” means adistance of air over which the wavelength of laser light is uniform inair.

OFDRs use a tunable wavelength high-coherence laser source. OFDR laserscan provide a wide optical frequency sweep which can translate to veryhigh spatial resolution in a reflectometer, but the cost of the lasersource is very high and suppliers are limited. A standard distributedfeedback (DFB) laser is much less expensive and can be tuned over asmaller wavelength range, but the DFB laser tunability is sufficient forresolution in the region of 1 cm or less. However, the DFB source doesnot typically provide sufficient coherence to be used for measurementsbeyond about 1 m of fiber length.

To support continued large-scale FO deployment, such as incommunications networks from homes to data centers, there is anidentified need for testing these FO installations at much lower cost,and with high resolution to localize faults in these more compactenvironments.

SUMMARY

In the view of the above, the present disclosure provides a newintegrated optics reflectometer which is able to further develop opticalcommunications at high reliability levels in a cost effective manner.

To support continued large-scale FO deployment for communications,sensing, advanced lighting systems and other FO platforms, theintegrated optics reflectometer of the present disclosure enables theoptical integrity of FO installations to be tested at high resolutionand much lower cost. The integrated optics reflectometer of the presentdisclosure provides inexpensive optical measurements with <1 cmresolution at a range of one or more kilometers. The integrated-opticsreflectometer provides a complete and/or customizable solution.

Therefore, to assist current and future FO installations operate at peakefficiency, the integrated optics reflectometer makes it easier fornetwork operators to quickly identify where fiber connectivity problemsexist. Likewise, one key area of improvement, addressed by the presentdisclosure, is the reduction in the cost of high-resolutioncommunication test and measurement devices. The integrated opticsreflectometer of the present disclosure supports large scale FO networkdeployment with a reflectometer having high resolution and moderatelyshort range to be used in the maintenance of FO networks. Thistechnology supports new FO networks being installed in neighborhoods,office buildings, fiber-to-the-home, local area networks, wide areanetworks, and mobile self-contained FO platforms including aircraft,ships, etc. Some of these FO platforms extend at most a few hundredmeters (<1 Km) and an optical interrogation system is desirable tolocate optical faults to within less than 1 centimeter to make necessaryrepairs to maintain overall optical network integrity.

The present disclosure provides that the laser output is monitored usingan interferometer (e.g., a measurement unit), and a feedback correctionis applied to the detected signal before the light is used in thereflectometer. The interferometer is utilized to measure theinstantaneous wavelength/frequency of the laser source, to determinewhether there is a deviation between an actual frequency of laser lightoutputted by the laser source at that instant and a target frequency ofthe laser source. The output of the interferometer can be considered tobe an “error signal” as it represents a deviation between the actualfrequency and the target frequency of the laser source. Based on ameasured deviation between the actual and target frequencies in theinterferometer, a feedback correction is applied in real time to thelaser source to adjust and correct the laser frequency to the targetfrequency, and thereby obtain a highly coherent lasing frequency outputfrom the laser source.

Accordingly, the present disclosure utilizes a phase/frequency controlfeedback mechanism based on lasing frequencies monitored with aninterferometer, to yield a corrected signal that works for all points ina long fiber (e.g., one or more kilometers) in one single measurement.High feedback correction gains are made possible by using an optimizedfeedback phase response to correct for laser fluctuations. The samefeedback mechanism also provides a point in which to inject a wavelengthcontrol signal to ensure control linearity and provide for accuratemeasurement results beyond the natural coherence length of the DFB lasersource.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional refinements, advantages and features of the presentdisclosure are described in more detail below with reference toexemplary embodiments illustrated in the drawings, in which:

FIG. 1A illustrates an arrangement of a laser source an a measurementunit (e.g., an interferometer);

FIG. 1B illustrates an arrangement of a laser source and a measurementunit (e.g., an interferometer);

FIG. 2 illustrates a block diagram of an integrated optics reflectometeraccording to an exemplary embodiment of the present disclosure;

FIG. 3 illustrates a MATLAB Simulink electro-optical model of awavelength managed DFB laser with a test fiber interferometer for lasersource interrogation;

FIG. 4 illustrates fiber Bragg grating (FBG) reflection intensity versuswavelength at three different temperatures;

FIG. 5 illustrates returned signals from three FBGs and a referencesignal from a wideband reflector;

FIG. 6 illustrates a graph showing the laser optical frequency is sweptversus time;

FIG. 7 illustrates simulation plots of the FO testing network device inFIG. 3 showing several key parameters including (from top to bottom)cavity electron number, cavity intensity, cavity optical phase, controlinterferometer output field and measurement interferometer output field;

FIG. 8 illustrates an example of post processing data from a test devicein order to numerically quantify and compare device performance; and

FIG. 9 illustrates a self-heterodyne configuration using an unbalancedMach-Zehnder interferometer.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure are described withreference to the drawings. The following description sets forth specificdetails, such as particular embodiments, procedures, techniques, etc.for purposes of explanation and not limitation. It is to be understoodthat the embodiments described hereinafter are exemplary, and otherembodiments may be employed apart from these specific details. In someinstances, detailed descriptions of well-known methods, interfaces,circuits, and devices are omitted so as not obscure the description ofthe present disclosure. Moreover, individual blocks are shown in some ofthe drawings. One skilled in the art will appreciate that the functionsof those blocks may be implemented using individual hardware circuits,using software programs and data recorded on a non-transitorycomputer-readable recording media, in conjunction with a suitablyprogrammed digital microprocessor or general purpose computer, usingapplication specific integrated circuitry (ASIC), and/or using one ormore digital signal processors (DSPs).

In the description of exemplary embodiments of the present disclosure,examples of types of laser sources are provided, such as diode lasersincluding DFB lasers. It is to be understood that these are given asexamples of laser sources, and the present disclosure is not limitedthereto.

At the outset of the detailed description of the present disclosure, adiscussion of the principles of OFDR is provided with reference tovarious examples to better illustrate the unique solution of the presentdisclosure.

Reflectometers are used to examine the characteristics of waveguidesalong their length, by analyzing reflections of a signal injected at oneend. In the OFDR, the signal is coherent light with a steadily changingoptical wavelength (or optical frequency).

In one OFDR example, the light may be steadily increasing in opticalfrequency at a rate of 10 MHz per microsecond from a nominal frequencyof 200 THz (1500 nm wavelength), and this frequency sweep may bemaintained for a duration of 1000 microseconds. Light which is reflectedfrom a discontinuity (e.g., a defect, connector, end face, etc.) in thefiber at a distance of 1 km will experience a round trip delay ofapproximately 10 us (using a typical value for the speed of light infiber). Thus, while the optical frequency is sweeping, the reflectedlight will be from the laser source at a time when the generated opticalfrequency was 100 MHz slower. This particular frequency differencebetween the reflected light and the current laser output frequency willbe a characteristic of all light from this distance for as long as thesweep is maintained. Light reflected from all other distances will havea different characteristic difference frequency.

Optical frequency/wavelength can be difficult to measure directly withhigh resolution, but very small frequency differences can readily bedetected between two light sources by interference. In this case, thereflected optical energy can be interfered with some of the light takenfrom the laser directly. This arrangement is sometimes calledself-heterodyning. The difference in optical frequencies between thesetwo interfering light signals produces an interference beat, orintensity modulation frequency equal to the optical frequency differenceof the two light input signals. The optical intensity modulation afterinterference can be converted into an electrical signal using aphoto-detector, and moving the signal to the electrical domain is usefulas we have better tools for further processing in this domain. Thereflection from a point 1 km distant now produces a signature 100 MHzelectrical signal in this arrangement. Its frequency identifies wherethe reflection came from, and the amplitude of the beat frequencyindicates the strength of the reflection at that point. It is apparentthat a frequency spectrum of the signals from the photo-detector nowcomprises an analog of the reflection profile of the fiber along itslength, where zero frequency represents the launch end and the fiberlength is represented on a scale where 100 MHz=1 km (i.e. 100 kHz/m).This frequency spectrum is often generated by a fast Fourier transform(FFT) of the beat signal from the detector.

In the above, it was assumed that the laser light is coherent. Thus, thelaser is an optical frequency oscillator where its phase can bepredicted for any length of time into the future. This, however, is notthe case in practice, as all oscillators are subject to somedisturbances and the oscillating phase of the semiconductor laser inparticular is disrupted by spontaneous emissions into its cavity. Whilestimulated photon emissions add to the lasing wave energy withconstructive phase, each spontaneously emitted photon has a random phaseand thus adds a small disturbance of random sign and amplitude to thelasing phase. In the course of time, the effect of these randomdisturbances is to cause the laser phase to wander away from the idealtarget phase (i.e. had it been perfectly coherent) in a “random walk”fashion. If the random contribution is very small, it might be possibleto predict the laser phase some useful way into the future. This validprediction time is the laser coherence time. It can also be expressed aslasing coherence length by dividing coherence time into the speed oflight. Thus, for example, a laser with coherence time of 3 μs has acoherence length in air of 1 km. Taking the example above and assuming alaser of coherence time 3 μs, the coherence length in fiber is about 600m (the speed of light being slower in fiber) so we could makereflectometer measurements to a distance of 300 m in fiber (i.e. 600 mround trip). A laser with longer coherence length could then be used tomake distant reflectometer measurements.

It is to be noted that the above discussion of coherence may imply thatthe laser phase is well defined to some time and beyond that time it isunknown, or that the coherence function has a high value to thecoherence length and quickly falls to nothing beyond that distance.However, the typical coherence function is Lorenzian in shape and thisis a statistical distribution that falls slowly and has a long tail. Thecoherence “cut-off” point may be defined as the point where the laserlight is 50% coherent, or where resulting interference fringes have 50%contrast, but the degree of coherence changes only slowly over a widerange. However, while a single figure for coherence length may besomewhat misleading, it is to be understood that such a figure isprovided for the purpose of quantifying coherence length/time.Accordingly, in describing the OFDR operation, the numbers used may beapproximated to a degree, but such approximation does not affect the useor understanding of such coherence lengths or times as discussed herein.

As described above, any physical point along the fiber length isidentified with a particular frequency after self-heterodyne detection.While this characteristic frequency is proportional to distance, it isalso proportional to the rate of change of lasing optical frequency. Inorder to maintain high resolution, exemplary embodiments of the presentdisclosure provide that the detected frequency from that point is madeconstant. If in the example above, it is desired to resolve thereflection at a point 1 km distant to 1 cm (1 part in 100,000), anoptical frequency sweep-rate linearity of 1 part in 100,000 would berequired. However, tunable lasers cannot readily be tuned with thisdegree of tuning linearity.

Measurement distance may be limited by laser coherence, as noted. Thereare three main techniques to overcoming this limitation: 1) by using ahigher coherence tunable laser source as described above, 2) bycompensating the raw measurement data for laser incoherence beforeinterpreting results, as disclosed, for example, in U.S. Pat. No.7,515,276 to Froggatt et al. (hereinafter “Froggatt”), or 3) bycorrecting the laser incoherence at the laser source, according to thesolution of the present disclosure as described in more detail below.

Technique 1) is a viable solution, and there are some high qualitytunable laser sources that can produce similarly high qualitymeasurement results with a measurement range of many km, but theselasers, which are often micromechanical structures including laserdevices combined with MEMS components and mirrors, are generallyexpensive to manufacture. By contrast, a common DFB semiconductor laserof moderate quality is manufactured at a much lower cost as an opticalsource for telecommunications systems. DFB lasers have a coherencelength that more typically permits OFDR measurements over a very limitedspan of approximately 1 m, and this is not often a useful range.

Technique 2) of Froggatt has been used as a means to allow inexpensiveDFB lasers to be used in making long length measurements. In principle,the technique of Froggatt involves measuring the laser phase variationswith time and recording the measurements electronically so that there isa historical record of the actual laser phase (or an approximation) thatcan be used to compensate the readings at the detector to some point inthe recent past. Accordingly, Froggatt proposes to monitor the DFB laserphase with an interferometer and then use that information, which hasbeen digitally delayed, to generate a historical record ofphase/frequency evolution, and then, based on that historical record,empirically compensate OFDR readings from a region of fiber beyond thelaser coherence length. The delay used for the laser monitor data isequal to the delay to the compensated region, as a correction term inactual reflectometer measurements at distances beyond the coherencelength of the laser, but the compensation only applies to a small windowsize around the compensated delay, and to view the whole fiber lengthone must make many partial readings.

The present disclosure provides a new technique of monitoring the laserphase evolution and using this information immediately in a feedbackloop to correct the laser phase variations and thereby increase thelaser coherence length/time. By correcting the laser behavior at thelaser source, the measurement data gathered will be applicable over thefull measurement length, as in the system using a highly coherenttunable laser source according to technique 1) above. The presentdisclosure provides that the laser output is monitored using aninterferometer, and a feedback correction is applied to the laser sourceto correct its optical frequency. Accordingly, the present disclosureutilizes a phase/frequency control feedback mechanism based on lasingfrequencies monitored with an interferometer, to yield a correctedsignal that works for all points in a long fiber in one singlemeasurement. High feedback correction gains are made possible by usingan optimized feedback phase response to correct for laser fluctuations.The same feedback mechanism also provides a point in which to inject awavelength control signal used to provide the optical frequency sweeprequired in order to use this DFB laser as an OFDR optical source.

FIG. 1A is a block diagram of an arrangement of a laser source 110 and ameasurement unit 120 (e.g., an interferometer). The laser source 110 hasa target frequency, but as noted above, the laser source does not alwaysoutput laser light at this target frequency. The measurement unit 120 isconfigured to measure a deviation between an actual frequency outputtedby the laser source 110 at a current instant in time and the targetfrequency of the laser source 110.

An example of an interferometric measurement on laser output to monitor(e.g., detect) the optical phase/frequency of the laser source 110 willnow be described with reference to FIG. 1A, to illustrate how thismonitoring is used to provide real-time feedback control for the lasersource 110.

Instantaneous laser phase and/or optical frequency can be monitoredusing an interferometer on some light output from the laser source 110.In a Mach-Zehnder interferometer, for example, of the measurement unit120, the laser light may be split into two paths, one short and onelonger, and then recombined. When the light is recombined, interferencetakes place and this may be constructive or destructive. In accordancewith an exemplary embodiment, the interferometer can be built from fiberand 2×2 fiber couplers. The first 2×2 coupler splits laser light intotwo fiber paths. One input of a second 2×2 coupler is fused directly toone output the first coupler, while the other input and output pair areconnected via an additional length of fiber. One or both of the outputsof the second 2×2 coupler may be connected to a photo-detector tomonitor the interference result. If the path length difference frominput to one output of this interferometer is an exact number ofwavelengths, then interference at that output will be constructive andthe light output will be maximum (approximately equal to the total inputlight level), while the path length difference at the other output ofthe interferometer will necessarily be one half-wavelength different (ifthese are equal split ratio 2×2 couplers this will be the case—theoptical delay difference may not be apparent but it occurs in thecouplers) and the second output level will be close to zero. If thewavelength changes such that the first path length is one halfwavelength longer than above, then the first output will be zero(destructive interference) and the second output will be maximum(constructive interference). At points in between the two conditionsabove, the intensity at either output will be a function of the lasingwavelength, and if the fiber length difference is many wavelengths oflight then the output intensity can be a very sensitive function ofoptical frequency. It is also taken into account that on a shortertimescale, short compared with the propagation delay time through thepath difference, the output intensity is a direct function of the laserinstantaneous change of phase as light from the longer path has not yetexperienced that phase change.

FIG. 1B illustrates a block diagram of an alternative embodiment inwhich the measurement unit 130 includes a recirculating interferometerinstead of the Mach-Zehnder interferometer illustrated in FIG. 1A.Nevertheless, the measurement units 120, 130 in FIGS. 1A and 1B are eachconfigured to measure a deviation between the frequency outputted fromthe laser source 110 at the current instant of time and the targetfrequency of the laser source 110.

FIG. 2 illustrates an exemplary embodiment of a an apparatus 200including a laser whose optical frequency is stabilized and thus itscoherence length increased according to the present disclosure. Theapparatus 200 utilizes the interferometer 120 of FIG. 1A to monitor thelasing frequency of the laser source 110, and the interferometricmeasurement is fed into a phase/frequency control feedback mechanism tocontrol in real-time the frequency/wavelength of the laser light outputby the laser source 110. Reference numeral 210 in FIG. 2 denotes afeedback control unit which is configured to, based on a measureddeviation between the actual and target frequencies of the laser source100 by the interferometer 120, control the laser source 110 in real timeto maintain a constant frequency of laser output from the laser source110 so that the frequency of laser light transmitted from the lasersource 110 is adjusted to the target frequency. The output of theinterferometer 120 can be considered to be an “error signal” as itrepresents a deviation between the actual frequency and the targetfrequency of the laser source 110. Based on a measured deviation betweenthe actual and target frequencies in the interferometer, the feedbackcontrol unit 210 applies a feedback correction in real time to the lasersource 110 to adjust and correct the laser frequency to the targetfrequency, and thereby obtain a highly coherent lasing frequency outputfrom the laser source 110.

As noted above, the time evolution of laser phase/frequency, as measuredby an interferometer, is based on reflected (delayed) signals which areself-heterodyned with the laser light generated at different instancesin time. In accordance with an exemplary embodiment of the presentdisclosure, the real-time measurement of laser frequency is usedimmediately in a feedback loop to instantly (e.g., by the infiniteimpulse response (IIR) filter illustrated in FIG. 2) correct thefrequency error between the target and actual frequencies of laser lightoutputted by the laser source 110.

This is achieved by providing two design characteristics. First, theoptical frequency detection and feedback loop must incur minimal delays,small compared to the bandwidth of corrections required. Thisnecessitates short fiber lengths (if fiber is used, else an integratedoptic implementation can be used to keep waveguide lengths small) usedin the interferometer and feedback detector(s) and high speed, lowlatency electronics used in the feedback loop. Second, there is provideda means of applying corrective tuning to the laser source 110 withsufficient bandwidth to correct frequency errors as they occur. DFBlasers are commonly tuned by change of temperature, e.g. using anattached Peltier cooler, but this has a time response on the scale ofseconds and is too slow for use in our feedback design. A DFB laser mayalso be tuned slightly by the current flowing in the laser (a change ofcurrent also modulates the light intensity from the laser) and this isthe mechanism applied in the present disclosure. The primary effect hereis temperature changes from the laser current heating, but there arealso more rapid changes in the electron number in the laser cavity whichaffects the refractive index and thus the lasing wavelength. The presentdisclosure exploits the mechanisms available to close the feedback loopand to minimize the disruptions to laser phase that otherwise result inlow coherence length. Some short term (on a scale of nanoseconds) laserphase noise can be tolerated, but the feedback loop restores laserfrequency quickly to maintain coherence on the timescale of microsecondsto tens of microseconds. This coherence characteristic makes the lasersuitable for the OFDR instruments.

It has been observed that tuning linearity is important for a highresolution instrument. Most tunable lasers (including the “high quality”devices mentioned above) have modest tuning linearity, but nonethelessare inadequate for many OFDR measurements. Lasers are tuned by adjustingtemperature, by adjusting angles of MEMS mirrors and gratings, etc.These “open-loop” mechanisms have some non-linearity. The solution ofthe present disclosure is to apply lasing frequency control within thefeedback loop discussed above.

The feedback stabilization technique of the present disclosure, asdescribed above, is effective in stabilizing phase at only one fixedfrequency. Frequency tuning can be added to this by varying the opticalphase at which the feedback loop locks. For example, a quadrature phasefeedback loop with digital phase rotation can be provided for precisiontuning.

One means of implementing this frequency control using the feedback loopinvolves tuning by stretching the interferometer's long fiber, whichworks as follows: The feedback loop is satisfied by some voltage at theoutput of the feedback photo-detector. This voltage is generated by acertain phase relationship between light in the short and long arms ofthe interferometer. If the fiber is stretched then in order to maintainthis phase relationship, the wavelength is increased so that the samenumber of wavelengths fit into the new fiber length with the same phaserelationship at the output coupler. If this is not achieved, then thedetector output voltage changes and the laser current changes to pullthe lasing frequency so that the condition is restored, by which thelaser wavelength will be adjusted to satisfy the new long fiber length.The fiber can be stretched by the small amount necessary to effect thistuning by wrapping it securely onto a Piezo cylinder, for example, andchanging the dimensions of the cylinder by an applied voltage. The lasertuning range is limited by the current tuning range that can be achievedin the laser source 110 used. The degree of stretching required to reachthis limit is rather small compared to the strain that can be withstoodby the fiber. This wavelength tuning mechanism was demonstrated in amodel illustrated in FIG. 3, which shows a MATLAB Simulinkelectro-optical model of a wavelength managed DFB laser with a testfiber interferometer for laser source interrogation.

According to an exemplary embodiment, another technique of tuningincludes rotating the phase of the feedback locking condition. Aquadrature interferometer output can be achieved by various means, forexample, by using an equal split 4×4 coupler for the output coupler ofthe interferometer. According to an exemplary embodiment, this couplercan have four outputs that represent combining the two light paths infour quadrature relationships. An orthogonal pair of these can have a 90degree relationship in their output intensity. By detecting both ofthese, a rectangular coordinate representation of the optical phaserelationship in the interferometer is obtained. These detector outputscan be summed in the appropriate (e.g., rectangular to polar mapping)ratio to satisfy any phase relationship when the feedback loop islocked. By rotating the rectangular to polar mapping, the lockingoptical phase can be made to rotate, and the laser wavelength willchange to satisfy the lock condition. Rotation can be through more than360 degrees, and each complete rotation will add another wavelength intothe interferometer imbalance length, ramping the lasing wavelength as itgoes. This phase rotation, and laser tuning, can continue until thelimit of the laser current tuning range is reached.

Accordingly, the feedback control unit 210 is configured to control thelaser source 110 to maintain a linear rate of change in the frequency ofthe laser light output from the laser source 110. A mapping of at leasttwo quadrature interferometer output signals are added to theinterferometer phase so that the feedback loop can be locked with anyoptical phase relation in the interferometer 120. As a result, themapping can be rotated to “wind” optical cycles/wavelengths into or outof the interferometer 120. The feedback loop of the feedback controlunit 210 adjusts the laser frequency to suit these changes to maintainfeedback lock, which results in sweeping the laser frequency withprecision.

Each wavelength added to the interferometer imbalance length representsa precision change in lasing wavelength and frequency, thus tuning isvery precise as represented by the count of wavelengths “wound” into orout of the interferometer. This may be implemented as a digital andprecise tuning mechanism. Interpolation between these integer wavelengthcounts, controlled by the rectangular to polar mapping, can be achievedusing multiplying DACs (digital to analog converters) and digitallycontrolled. While this is not naturally as digitally precise as countingwavelengths, it can be designed to be accurate and the digitalcoefficients can be readily compensated to increase precision. Theoverall result of this technique is a laser that can be tuned (withinits current tuning range) with very high digital precision that issufficient for many OFDR applications without requiring further signalprocessing or compensation methods. By achieving this, then thecombination of laser and feedback mechanism can be used (e.g., as ablack box) as being equivalent to a high coherence laser source that canbe used to make longer range OFDR measurements directly without furthercompensation, and with results in each measurement that are valid overthe full length of measurement. The remainder of the design to completean OFDR can thus be quite simple, straightforward, and low cost.

As described above, quadrature information can be generated from theinterferometer using a 4×4 coupler. The same result can be achieved byother means, for example, a 4×4 coupler provides an approximation to aquadrature output, but it would, relative to the 4×4 coupler, involve amodified mapping between its output and the phase to be locked. Inaddition, in the integrated optics reflectometer, the 4×4 coupler may becomposed of other elements. For example, it could be built from 2×2elements. The 4×4 coupler may be composed of a multimode fiber sectionwith its output illuminating a segmented photodiode so long as in eachcase, the electrical output from the detectors is mapped in advance tochanges in optical phase of the two interfering paths. Documentsdescribing this include Travis, ARL et al. “Passive quadrature detectionusing speckle rotation on a multisegment photodetector,” Optical FiberCommunication Conference and Sixth International Conference onIntegrated Optics and Optical Fiber Communication (1987), the contentsof which are incorporated herein.

It may be desirable to achieve a wider tuning range than can be achievedby current tuning alone. In accordance with an exemplary embodiment,temperature tuning can be added inside the feedback loop. Temperaturetuning is not inherently sufficiently linear for tuning the OFDR laserwavelength. However, if it is added to a laser that is inside awavelength control feedback loop as described above, the feedback loopcontrol (current tuning) can generate the correction terms needed tolinearize the laser tuning to achieve the same high degree of linearity(as the purely current tuning mechanism above) with a wider tuning rangeassociated with temperature tuning.

One limitation of temperature tuning is a relatively slow response fortemperature changes in response to the heating and cooling input. Inaccordance with an exemplary embodiment, Magnetic Refrigeration can beutilized to change the temperature of the laser chip more rapidly thancommon Peltier effect devices. Magnetic Refrigeration is a phenomenonwhere the temperature of a material can be changed by the application ofa magnetic field. When the field is removed the temperature reverts tothe previous value. There is no heat generated or removed, i.e. theprocess is adiabatic. This implies an “instantaneous” temperature changefor an instantaneous change in magnetic field strength. By attaching thefeedback stabilized laser to a piece of Magnetic Refrigeration material(e.g, Gadolinium (Gd) or its alloys), then the laser temperature mightbe changed very rapidly, though not instantly as heat energy still needsto be exchanged between the Magnetic Refrigeration material and thelaser chip to change its temperature.

Another technique for changing the temperature of the laser chip morerapidly than common Peltier effect devices relies on the electrocaloriceffect of certain materials. This is a phenomenon where a material showsa reversible temperature change when an electric field is applied to thematerial. It is theorized that a voltage passing through the materialraises or lowers the entropy of the system which results in atemperature change although knowledge of the physical mechanism is notnecessary to an understanding of the presently described integratedoptics reflectometer. This effect is the physical inverse of thepyroelectric effect where materials generate a voltage when heated orcooled, and is also distinguishable from the thermoelectric effect wherea temperature difference is created when current is driven through anelectric junction with two conductors. Documents describing theelectrocaloric effect include Scott, J. F. “Electrocaloric Materials,”Annual Review of Materials Research, Vol. 41: 229-240 (August 2011), A.S. Mischenko, et al. “Giant Electrocaloric Effect in Thin-FilmPbZr0.95Ti0.05O3,” Science, Vol. 311, no. 5765, 1270-1271 (Mar. 3,2006), and Bret Neese, et al. “Large Electrocaloric Effect inFerroelectric Polymers Near Room Temperature,” Science, Vol. 321, no.5890, 821-823 (Aug. 8, 2008), the contents of which are incorporated byreference herein.

An exemplary embodiment of the present disclosure provides a mechanismto temperature tune the laser source 110 with a temperature adjustmentcomponent including a Magnetic Refrigeration material to adjust thetemperate of the laser source 110. The temperature adjustment componentcan rapidly tune the temperature of the laser source 110, independent ofor in conjunction with the feedback control unit 210. Accordingly, anexemplary embodiment of the present disclosure also provides anapparatus which includes a laser source (e.g., the laser source 110 ofFIGS. 1A and 1B) and a temperature adjustment component including aMagnetic Refrigeration material or electrocalorific material to adjustthe temperature of the laser source and thereby control the frequency ofthe laser light output from the laser source.

Exemplary embodiments of the present disclosure have been described asbeing configured to output laser light into an optical component such asa FO installation. However, the present disclosure is not limitedthereto. The embodiments of the present disclosure are also applicableto free-standing or stand-off implementations in which laser light istransmitted toward an area of interest through any medium, such as air,for example. As used herein, an optical component can be any one or moreof an optical fiber, an optical coupler, an optical connector, anoptical switch, an optical integrated optical waveguide, liquid,atmosphere and free space.

An exemplary embodiment of the fiber construction was described above toinclude, for example, 2×2 couplers. The present disclosure is notlimited thereto. For instance, an integrated optics solution can beprovided where the laser, couplers, and detectors are integrated onto asmall substrate.

Accordingly, exemplary embodiments of the present disclosure combine anelectro-optical controller with a standard DFB laser source to managethe wavelength to facilitate an optical frequency sweep with definedcoherence that can be used to make reflectometer measurements over longfiber lengths (up to a few km, e.g., 5 km) in one measurement, withresolution in the region of a few mm. The laser control configuration ofthe present disclosure is modeled using a modeling system that hasdemonstrated proven accuracy in other optical systems with promisingresults in FIG. 3.

The reflectometer technique of the present disclosure, which combines anelectro-optical controller with a laser source such as a DFB, managesthe output wavelength thus facilitating an optical frequency sweep withdefined coherence that can be used to make reflectometer measurementsover long fiber lengths, e.g., one or more kilometers, with resolutionof several mm. Thus, the present disclosure, using optical modelingtechniques to evaluate and design DFB line width stabilization, allowswavelength tuning designs with very long coherence time and a wavelengthtuning range from a regular telecommunications grade laser source.Accordingly, an exemplary embodiment of the present disclosure providesa laser source configured to output laser light having a coherencelength of at least about a hundred meters.

This new reflectometer technology will be able to be deployed as ahand-held/integrated measurement/demodulation tool forneighborhood/office/government FO networks (e.g., WANs, LANs, etc.), aswell as mobile self-contained FO platforms including aircraft, ship,terrestrial and other vehicles, and FO sensing. Many of these FOplatforms extend at most a few hundred meters (<1 km) and an opticalinterrogation system is desirable to accurately locate optical faults(within <1 cm) to make necessary repairs maintaining overall FO systemintegrity not available with current techniques.

Agencies of the U.S. government, for example, uses FO technology for awide variety of air, sea, ground, and space applications. FOapplications can be highly specialized, often with very specific projectand scope requirements requiring rigorous testing and harsh environmentcertification to ensure reliability and performance in the field. Forexample, aerial vehicles are a rapidly growing application for FO.Utilized as the communications link between ground control and theantenna controlling the UAV, FO provides a very fast and efficient meansfor transmitting extensive data over long distances. Typically, aerialvehicle FO links for vehicle positioning/control and information/datatransmission with high bandwidth and rapid transmission over longdistances. For ground vehicles, automotive applications, FO are anaturally ideal choice for lighting, communications, and sensingrequirements. Lastly, for ship systems, FO systems have been deployedfor mission critical on-board communication systems and ship to shorelinks to provide data, phone, and other services to docked ships. Theseconnections allow high speed, high bandwidth communications to and fromthe vessel, without using shipboard wireless transmit/receive systems.

The present disclosure will also serve as the interrogation system forcoherent or interferometric measurements such as distributed sensorsystem using fiber Bragg gratings (FBGs), extrinsic Fabry-Perotinterferometers (EFPIs), or other transducers. In a typical distributedfiber sensor system, it is necessary to first provide a means to measuretemperature, pressure, stress/strain and vibration in an optical fiber,and second, it is also necessary to provide a means to multiplex severalmeasurements onto one fiber. For example, the wavelength managed DFBlaser used with FBG sensor can satisfy both requirements.

Firstly, the FBG can be designed to reflect light at a wavelength withinthe sweep range of the laser, with the reflection wavelength determinedby the FBG grating period. Due to strain or changes in temperature thefiber dimensions may change, and/or the core refractive index maychange, which modifies the reflected wavelength, as shown in FIG. 4,which illustrates FBG reflection intensity versus wavelength at threedifferent temperatures. By measuring the peak wavelength reflected fromthe fiber, the strain or temperature at the FBG can be determined. Thisprocess requires a tunable laser source, but does not necessarilyrequire high source coherence.

If the source wavelength is swept rapidly, then it may be possible todetermine the wavelength difference between the laser at the currentpoint in time and any FBG reflected signal which will have thewavelength seen at the source before the round trip delay time to theFBG as shown in FIG. 5, which illustrates returned signals from threeFBGs and a reference signal from a wideband reflector. This wavelengthdifference may be a tiny fraction of the laser wavelength, but may stillamount to kilohertz or megahertz in frequency terms, and this frequencyrange is readily captured and processed electronically. The frequencydifference can be detected using coherent optical detection (e.g. bymixing the two light signals on a photo-detector) to produce a beatfrequency. Each of the FBGs, located at different points in the fiber,will have a unique round trip distance hence a unique beat frequencyshown in FIG. 6, which illustrates the laser optical frequency is sweptversus time. Delayed reflections from distant FBGs exhibit an earlieroptical frequency which is offset from that of the nearby referencereflector. This system requires a coherent light source to facilitatecoherent detection, where the coherence may be (though not necessarily)at least as long as the round trip distance in fiber to the furthestFBG.

While FBGs are used above as an example of an in-fiber transducer thereare other possible transducers including EFPIs and even the fiber's ownintrinsic backscatter signature. The present disclosure will assist thenumerous optimizations to be made to gain best performance taking intoaccount the expected laser tuning range and speed, the particulartransducer sensitivity coefficients, the desired system measurementranges and sensitivities, and the desired measurement spatial resolutionand total length.

For sensing applications for the energy market, there is a desire forproviding high-resolution strain, temperature, vibration and pressurefor the energy generation and distribution industries. Fiber opticsensing systems have been developed to address these markets due totheir high level of accuracy and ability to operate in harshenvironments even at high temperatures and high pressures. Additionally,transformers, power companies' biggest capital investments, havetraditionally been managed manually, retroactively and inefficiently.Next-generation sensors deployed at substations will help utilitiesavoid unplanned failures, reduce maintenance costs and extend usefultransformer life. Wind energy sensing products are being developed thatallow for operational monitoring, tracking the condition of the windgeneration system. These new sensing applications are being addressed byFO sensing systems which monitor pitch to maximize generator output andload reduction for blades, drive train and tower, while giving costreduction benefits for the turbine manufacturer. For mobile platforms,health monitoring of airplanes, ships, spacecraft, etc. as well asstrain, temperature and other physical parameters will be measured andmonitored providing for safer, more efficient transportation systems ofthe future.

In accordance with the present disclosure, the integrated opticsreflectometer will be able to manage the output wavelength of the lasersource, thus facilitating an optical frequency sweep with definedcoherence that can be used to make reflectometer measurements over longfiber lengths, up to around five kilometers, in one measurement, withresolution in the region of several mm. The laser control configurationhas been modeled using a highly specialized optical modeling system thathas demonstrated proven accuracy in other optical systems. Using theseunique tools, the present disclosure improves upon past DFB line widthstabilization and wavelength tuning designs achieving very longcoherence time and a small (but sufficient) wavelength tuning range froma regular telecommunications grade laser source.

FIG. 7 shows simulation plots of the FO testing network device in FIG. 3showing several key parameters including (from top to bottom) cavityelectron number, cavity intensity, cavity optical phase, controlinterferometer output field and measurement interferometer output field.While not shown here, also monitored will be electrical circuitwaveforms (e.g. laser and detector current and voltage).

For the integrated optics reflectometer, an effective light sourceachieving the best performance from a communications grade DFB laser isutilized. Therefore, figures of merit with which to compare lightsources will be performed in order to fine tune the device. One suchquantitative figure of merit is laser line width. We can easily measurethis by post processing data taken from the model where we can readoptical phase directly, and thus can derive a fast Fourier transform(FFT) from this information.

FIG. 8 shows an example of post processing the data from theaforementioned test device in order to numerically quantify and comparedevice performance. In this case, an FFT of the laser cavity phase givesa measure of the laser line width, which will be reduced to anacceptable value. Data manipulation is well catered for in the Simulinkand MATLAB environment where the key to efficient development is anaccurate model which is validated against practical, measureable cases.

As noted above, with the physical light source, it is not so easy tomeasure optical phase directly, and so the laser line width cannot bemeasured directly. Instead a heterodyne of the laser light with adelayed copy of itself, e.g., self-heterodyne, is used to measure ofphase noise. If this is done with a very long delay, well beyond thecoherence length of the light source, then the line width measured is asif taking the product of two different sources with the same line widthand easily estimate the line width of one of them. However, this canrequire a very long fiber delay if coherence is fairly high, andfurthermore because the typical Lorentizian coherence distribution has avery long tail. Instead, we can use a shorter interferometer tointerfere the light with a slightly delayed version of itself andthereby get a measure of phase noise. Fortunately this is a principlethat can be modeled accurately and also realized in practice.

In FIG. 9, a self-heterodyne configuration using an unbalancedMach-Zehnder interferometer is shown, somewhat simplified because inpractice it is necessary to ensure polarization matching at the secondoptical coupler when there is a potential for polarization rotation inthe fibers. In the model it is easy enough to ensure this polarizationmatching by design.

The integrated optics reflectometer of the present disclosure provides alow-cost instrument platform including a reflectometer core module forFO instrumentation. The platform can be integrated into a hand helddevice with packaging and displays for installation and maintenancecrews. The platform can also be used as an integrated part of FO networkcontinuous monitoring systems. The present disclosure provides thenetwork operators with needed data in summary format to facilitatereporting of problems, and provide more detailed reports to providemaintenance crews specific information needed to fix problems. With thehigh resolution of the present disclosure, they can pinpoint issueswithin an individual connector or optical component. With the very lowcost, integrated optics reflectometer can be integrated across aResearch and Education Network (REN), small office/home office (SOHO),LANs, buildings and other markets in multiple locations to ensurenetwork monitoring and reliability. For the first time the presentdisclosure provides needed instruments at a cost point where they can bedesigned into new and retrofit into existing networks.

Many of these new FO network platforms are being developed and servicedby small and medium-sized technology companies that have limitedresources and capabilities. In addition to the larger customers, when itcomes to FO network testing small and medium enterprises (SMEs) faceunique challenges. Existing testing equipment in the opticaltelecommunications market is focused on serving large companies, whilethese SMEs are underserved. The present disclosure will provide thisunderserved customer, as well as large customers, with a FO testingsolution option at an affordable price while also being available.

The present disclosure satisfies several market desires. For instance,the present disclosure provides an inexpensive instrument developmentwith high resolution (<1 cm resolution at km range) with low perinstrument manufacturing cost. The present disclosure also provides ahighly flexible and configurable optical network interrogation platformwill eliminate the overwhelming myriad of test equipment options andwill be compatible with all currently available FO sensing elements(EFPI, FBG, LPG, etalon, dispersion, etc.). In addition, the presentdisclosure provides either a hand-held configuration or a simpleoptoelectronics card that can be integrated into the network formonitoring faults and performance as well as sensing systems. Internetdownloadable software upgrades can be utilized to keep the equipment atthe leading edge of performance.

The present disclosure also provides a method for performing theoperative functions of the integrated optics reflectometer as describedherein, as well as the uses of the integrated optics reflectometer in anOFDR instrument to measure fiber characteristics.

It will be appreciated by those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restricted. The scope of the invention is indicated by theappended claims rather than the foregoing description and all changesthat come within the meaning and range and equivalence thereof areintended to be embraced therein.

What is claimed is:
 1. An apparatus comprising: a laser sourceconfigured to output laser light at a target frequency; a measurementunit configured to derive a measured deviation between an actualfrequency of the laser source at a moment in time and the targetfrequency of the laser source, the measurement unit further comprising:first and second laser light paths dividing the laser light outputted bythe laser source, the first and second laser light paths havingdifferent lengths; and a detector measuring an interference pattern;wherein the interference pattern is generated when laser light travelingalong the first laser light path is recombined with laser lighttraveling along the second laser light path; wherein the measurementunit derives the measured deviation between the actual and targetfrequencies based on the measured interference pattern; and a feedbackcontrol unit configured to, based on the measured deviation between theactual and target frequencies, control the laser source to maintain aconstant frequency of laser output from the laser source so that thefrequency of laser light transmitted from the laser source is adjustedto the target frequency of the laser source.
 2. The apparatus of claim1, wherein feedback control unit includes an infinite impulse response(IIR) filter configured to compensate for characteristics of themeasurement unit utilized for frequency measurement.
 3. An OFDRarrangement comprising: the apparatus of claim 1, wherein: the OFDRarrangement is configured to output laser light from the laser source toa point in an optical component to be tested distant from the lasersource, and measure a difference in frequency between the outputtedlaser light and an instantaneous frequency of laser light reflected fromthe point, and the feedback control unit is configured to maintain aconstant distance frequency between the frequency of the laser lightoutput from the laser source and the instantaneous frequency reflectedfrom the point such that the difference in frequency is directlyproportional to the distance from the laser to the point, and anamplitude of the reflected laser light is proportional to thereflectivity at the point, to characterize a reflectivity profile of theoptical component.
 4. The OFDR arrangement of claim 3, wherein theoptical component is at least one of an optical fiber, an opticalcoupler, an optical connector, an optical switch, an optical integratedoptical waveguide, liquid, atmosphere and free space.
 5. The OFDRarrangement of claim 4, wherein the reflectivity profile of the opticalfiber is measurable at a length beyond a reflectivity profile measurableby the laser source not being controlled by the feedback control unit.6. The OFDR arrangement of claim 4, wherein the reflectivity profile ofthe optical fiber is measurable in a single measurement at a distancegreater than 1 m.
 7. The apparatus of claim 1, wherein the feedbackcontrol unit is Configured to stabilize a range of actual frequencies tothe target frequency of the laser source.
 8. The apparatus of claim 1,wherein the controlling of the laser source includes adjusting atemperature of the laser source by using a magnetic refrigerationmaterial.
 9. The apparatus of claim 1, wherein the controlling of thelaser source includes adjusting a temperature of the laser source byusing an electrocalorific material.
 10. An apparatus comprising: a lasersource configured to output laser light with increased coherence length;a measurement unit configured to derive a measured deviation betweeninstantaneous laser optical phase of the laser source at a moment intime and a phase reference taken from the same laser at an earlier time,the measurement unit further comprising: first and second laser lightpaths dividing the laser light outputted by the laser source, the firstand second laser light paths having different lengths; and a detectormeasuring an interference pattern; wherein the interference pattern isgenerated when laser light traveling along the first laser light path isrecombined with laser light traveling along the second laser light path;wherein the measurement unit derives the measured deviation between theinstantaneous laser phase and a target phase based on the measuredinterference pattern; and a feedback control unit configured to, basedon the measured deviation between the actual and target phase, controlthe laser source to reduce optical phase shifts with time and therebyextend coherence time and coherence length.
 11. The apparatus of claim10, wherein feedback control unit includes an infinite impulse response(IIR) filter configured to compensate for characteristics of themeasurement unit utilized for phase measurement.
 12. An OFDR arrangementcomprising: the apparatus of claim 10, wherein: the OFDR arrangement isconfigured to output laser light from the laser source to a point in anoptical component to be tested distant from the laser source, andmeasure a difference in phase between the outputted laser light and aninstantaneous phase of laser light reflected from the point, and thefeedback control unit is configured to maintain a constant distancephase between the phase of the laser light output from the laser sourceand the instantaneous phase reflected from the point such that thedifference in phase is directly proportional to the distance from thelaser to the point, and an amplitude of the reflected laser light isproportional to the reflectivity at the point, to characterize areflectivity profile of the optical component.
 13. The OFDR arrangementof claim 12, wherein the optical component is at least one of an opticalfiber, an optical coupler, an optical connector, an optical switch, anoptical integrated optical waveguide, liquid, atmosphere and free space.14. The OFDR arrangement of claim 13, wherein the reflectivity profileof the optical fiber is measurable at a length beyond a reflectivityprofile measurable by the laser source not being controlled by thefeedback control unit.
 15. The OFDR arrangement of claim 13, wherein thereflectivity profile of the optical fiber is measurable in a singlemeasurement at a distance greater than 1 m.
 16. The apparatus of claim10, wherein the feedback control unit is configured to stabilize a rangeof actual phases to the target phase of the laser source.
 17. Theapparatus of claim 10, wherein the controlling of the laser sourceincludes adjusting a temperature of the laser source by using a magneticrefrigeration material.
 18. The apparatus of claim 10, wherein thecontrolling of the laser source includes adjusting a temperature of thelaser source by using an electrocalorific material.
 19. The apparatus ofclaim 10, wherein: the OFDR arrangement is configured to output laserlight from the laser source to a point in an optical component to betested distant from the laser source, and measure a difference infrequency between the outputted laser light and an instantaneousfrequency of laser light reflected from the point, and the feedbackcontrol unit is configured to maintain a constant distance frequencybetween the frequency of the laser light output from the laser sourceand the instantaneous frequency reflected from the point such that thedifference in frequency is directly proportional to the distance fromthe laser to the point, and an amplitude of the reflected laser light isproportional to the reflectivity at the point, to characterize areflectivity profile of the optical component.